Unless I misread what was written, the apps listed above won’t suit your use case, as you’re looking for a way for customers to physically draw the note while on your online store, as opposed to just uploading an image of a pre-written signature (I can’t imagine most people have one of these sitting around ready-to-go).
It also doesn’t seem like there are any apps for this in the Shopify App Store, as everything that comes up for “handwritten notes” is intended for stores to send handwritten notes to their customers, not vice-versa.
While I’m not aware of any apps that implement this, here’s the approach you should take if you want build a custom app for allowing customers to attach handwritten notes to their orders.
You’ll need to add some code to your product page that uses the signature_pad JS library to allow your customers to draw signatures in their browser.
Once a signature is collected, render it to an SVG, and add it to a line item’s properties.
Check out this tutorial by a Shopify Community Manager to learn how to add custom fields, like this signature field, to your product pages:
https://community.shopify.com/post/616525
You can then view these line item properties on orders in your backend.
Let me know if you have any questions.